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Planet “lighter than cork” baffles astronomers

Sept. 14, 2006
Courtesy Harvard-Smithsonian Center for Astrophysics
and World Science staff

A new­found plan­et es­ti­mat­ed to be light­er than a gi­ant ball of cork is baf­fling as­tro­no­mers. They’re gues­sing that some un­known me­cha­nism heats this type of world in­ter­n­al­ly, puf­fing it up.

Artist's con­cept of HAT-P-1. (Cour­te­sy Da­vid A. Aguilar, CfA)


Us­ing a net­work of small au­to­mat­ed te­l­e­s­copes, re­search­ers at the Har­vard-Smith­so­ni­an Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., iden­ti­fied the pla­n­et. De­s­ig­nat­ed HAT-P-1, they said it or­bits one of a pair of dis­tant stars in the di­rec­tion of the con­stel­la­tion La­cer­ta.

“We could be look­ing at an en­tire­ly new class of pla­n­ets,” said the cen­ter’s Gas­par Ba­kos, who de­signed and built the tele­scopes, known as the HAT net­work. He is al­so the lead au­thor of a pa­per sub­mit­ted to the As­tro­phys­i­cal Jour­nal de­scrib­ing the find. 

About 1.38 times wi­der than Ju­pi­ter, HAT-P-1 is the big­gest known plan­et, yet weighs on­ly as much as half of Ju­pi­ter, the Bakos added. That would make it one fourth as dense as wa­ter. 

“In oth­er words, it’s light­er than a gi­ant ball of cork! Just like Sat­urn, it would float in a bath­tub if you could find a tub big enough to hold it, but it would float al­most three times high­er.” 

It circles its host star eve­ry 4.5 days in an or­bit one-twentieth of the  Earth-Sun dis­tance, the as­tronomers said. Once per or­bit, they added, it passes be­fore its par­ent star. This dims it by about 1.5 pe­rcent to our view. Pla­nets that dar­ken their parent stars in this way are called transiting planets. 

This star is part of a double-star sys­tem called ADS 16402, vis­i­ble with bin­oc­u­lars.

Al­though pe­rhaps strang­er than any known world out­side our so­lar sys­tem, HAT-P-1 is not alone in its low den­si­ty, re­search­ers said: the first tran­si­ting plan­et ev­er found, HD 209458b, al­so is puffed up about 20 pe­rcent larg­er than the­o­ry pre­dicts, while the new one is 24 pe­rcent larg­er.

“Out of 11 known trans­iting plan­ets, now not one but two are sub­stan­tial­ly big­ger and low­er in den­si­ty than the­o­ry pre­dicts,” so the pre­vi­ous find can no long­er be dis­missed as a fluke, the cen­ter’s Rob­ert Noyes said.

“This new dis­cov­ery sug­gests some­thing could be mis­sing in our the­o­ries of how plan­ets for­m,” he con­tin­ued. One way to ex­plain the find is that some un­known mech­an­ism pro­vides cer­tain plan­ets with more in­ter­nal heat than pre­vi­ously be­lieved, he added.

The HAT net­work con­sists of six tele­scopes that con­duct robotic ob­ser­va­tions each clear night. They seek out tran­sit­ing plan­ets. As­tronomers meas­ure a plan­et’s size from the amount of the star’s dim­ming.

Com­bined with the mass—de­ter­mined by meas­ur­ing the amount of the star’s wob­ble as the plan­et or­bits it—re­search­ers then cal­cu­late a plan­et’s den­si­ty. The new­found world is es­ti­mat­ed to be 450 light-years away; a light-year is the dis­tance light trav­els in a year.


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